help me help me
...sounds like a vacuum leak, do take it to have the the trouble codes read, your parts store should be able to do this for no money, then we's start to get a clue. help us help us by providing a bone to chew on.
When you get your trouble codes read, ask them to check the RPM output of the ECU. If that output is steady, your gauge is bad.
Search Cadillac Seville Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Cadillac Seville Experts